1. Field of the Invention
The present invention relates to an image processing method for eliminating noise components from a color image signal, and more particularly to an image processing method for producing a color image wherein image portions exhibiting a specific color, e.g. the color of human skin or the color of the sky, are not degraded by noise components and the color quality of other objects within the image regions exhibiting the specific color is not lost.
2. Description of the Prior Art
There are known various methods of spatial image processing for eliminating random noise components from an original image signal. For example, in one such method the original image signal containing noise components is processed so as to uniformly suppress its high frequency components. Although this method effectively eliminates the noise components, the resulting image lacks sharpness and gives a fuzzy impression. For overcoming this disadvantage there is also known a processing method wherein a low-frequency component and a high-frequency component are separated from the original, a low-level signal including noise components is eliminated from the separated high-frequency component, and the low- and high-frequency components are then recombined. Although the image produced using the image signal processed by this method is free of noise and exhibits good sharpness, it tends to lack subtlety and gives the impression of a watercolor painting.
For eliminating this problem there have been suggested methods such as that disclosed in Japanese Unexamined Patent Publication No. 58(1983)-14270, which relates to a method of image processing by using one set of processing conditions for specific regions within the image and a different set of processing conditions for the other regions. As this method makes it possible to effectively retain edge portions of images, it is capable of preventing the aforesaid problem. On the other hand, however, when it is applied for the production of color images including regions representing the human skin, a type of color image that frequently has to be handled in the making of photographic prints and in color printing, there arises a new problem.
More specifically, in a color image including regions representing the human skin, it is these flesh colored portions that are affected most strongly by noise-induced degradation. Thus when the aforesaid method is applied, the flesh colored regions are designated as the specific regions with respect to which the processing for noise elimination is carried out. However, within these flesh colored regions there often exist numerous non-flesh colored objects. For example, in the case of the human face, there will be such non-flesh colored portions as those corresponding to the hair, the eyebrows, the eyes, dimples, wrinkles and possibly even minute objects in the foreground. When the aforesaid image processing is carried out, such objects within the specific regions become smudged and lose their texture.